Shape Memory Alloy Elements Research Articles

Two-Way Coupled Statement of the Problem of Loss of Stability due to Inverse Thermoelastic Phase Transition in a Shape Memory Alloy

A two-way coupled statement of stability problem for shape memory alloy elements is given in the framework of the “fixed load” and “variable load” concepts. It is shown that the largest values of the critical parameters are obtained when solving the problem in the two-way coupled statement in the framework of the “fixed load” concept and the least values are obtained in the oneway coupled statement in the framework of the “variable load” concept.

Microstructure, Mechanical Property, and Phase Transformation of Quaternary NiTiFeNb and NiTiFeTa Shape Memory Alloys

Based on ternary Ni45Ti51.8Fe3.2 (at %) shape memory alloy (SMA), Nb and Ta elements are added to an NiTiFe SMA by replacing Ni element, and consequently quaternary Ni44Ti51.8Fe3.2Nb1 and Ni44Ti51.8Fe3.2Ta1 (at %) SMAs are fabricated. The microstructure, mechanical property, and phase transformation of NiTiFeNb and NiTiFeTa SMAs are further investigated. Ti2Ni and β-Nb phases can be observed in NiTiFeNb SMA, whereas Ti2Ni and Ni3Ti phases can be captured in NiTiFeTa SMA. As compared to NiTiFe SMA, quaternary NiTiFeNb and NiTiFeTa SMAs possess the higher strength, since solution strengthening plays a considerable role. NiTiFeNb and NiTiFeTa SMAs exhibit a one-step transformation from B2 austenite to B19’ martensite during cooling, but they experience a two-step transformation of B19’-R-B2 during heating.

Adaptive tuned vibration absorber based on magnetorheological elastomer-shape memory alloy composite

Abstract Shape memory alloy (SMA) is an attractive smart material which could be used as stiffness tuning element in adaptive tuned vibration absorber (ATVA). The sharp modulus change in SMA material during phase transformation creates difficulties for smooth tuning to track forcing frequency to minimize vibrations of primary system. However, high hysteresis damping at low temperature martensitic phase degrades performance of vibration absorber. This paper deals with the study of dynamic response of system in which SMA and magnetorheological elastomer (MRE) are combined together to act as a smart spring- mass-damper system in a tuned vibration absorber. This composite is used as two way stiffness tuning element in ATVA for smooth and continuous tuning and to minimize the adverse effect at low temperature by increasing equivalent stiffness. The stiffnesses of SMA element and MRE are varied respectively by changing temperature and strength of external magnetic field. The two way stiffness tuning ability and adaptivity have been demonstrated analytically and experimentally. The experimental results show good agreement with analytical results. The proposed composite is able to shift the stiffness consequently the natural frequency of primary system as well as reduce the vibration level of primary system by substantial mount.

Using 0–1 test to diagnose chaos on shape memory alloy dynamical systems

Shape Memory Alloy (SMA) dynamical systems may exhibit a rich response that can include periodic, quasi-periodic, chaotic and hyperchaotic behaviors. In this regard, diagnostic tools are important in order to identify the different types of behaviors. This paper aims to analyze systems with SMA elements through a nonlinear dynamics perspective with a specific focus on the use of 0–1 test to quantify the chaoticity of the dynamical response of SMA oscillators. The investigation includes different constitutive models for the restitution force on both single- and two-degree of freedom oscillators. Results of the 0–1 test are compared with Lyapunov exponents calculated with different algorithms. The analyses show that the 0–1 test can be considered a reliable and computationally efficient alternative as a diagnostic tool of chaotic responses.

Modeling and parametric studies of magnetic shape memory alloy–based energy harvester

This article presents a model to simulate the behavior of a magnetic shape memory alloy while harvesting vibratory energy. In this type of energy harvester, magnetic shape memory alloy element is placed in the air gap of a ferromagnetic core which conducts the magnetic flux. Two apparent coils are wound around a ferromagnetic core: one to produce bias magnetic field by passing a rectified electric current and the other to serve as an energy pickup coil. Applying compressive time-variant strain field to magnetic shape memory alloy element changes its dimensions and magnetic properties as well. Presence of the bias magnetic field returns magnetic shape memory alloy element to its initial state while removing the load. Changes in magnetic properties of magnetic shape memory alloy will change the magnetic flux passing through the pickup coil and will produce an alternative voltage in that coil based on the Faraday’s law of induction consequently. In the modeling strategy, magnetic behavior of magnetic shape memory alloy element during operation is obtained by solving the thermodynamic-based constitutive equation numerically and the energy harvesting process modeled with equivalent magnetic and electric circuits. By varying the system parameters in the model, output voltage and power changes are reported accordingly.

Experimental Validation of Condition Monitoring for Electrically Activated Shape Memory Alloys for an Unlocking Device

Shape memory alloys (SMA) belong to the group functional materials which can be activated thermally. Along with a phase transformation, they can remember a previously imprinted shape and have a special resistance behavior. Therefore, they can also be used as a sensor and may be capable of detecting various system states in technical systems. This paper makes a contribution by evaluating the measurability of measured variables by SMA elements. Furthermore, it investigates the technically relevant states of “blockade” and “activation” of electrically activated shape memory actuators. It develops and validates an algorithm which is able to detect a possible “blockade”. Moreover, this work presents a hardware concept for a condition monitoring system of shape memory actuators.

Finite element simulation of ferromagnetic shape memory alloys using a revised constitutive model

In this article, a previously developed constitutive model for ferromagnetic shape memory alloys is phenomenologically enhanced using experimental observations. A modified phase diagram along with a method for calibration of the required model parameters is further presented. The model is implemented into a user material subroutine to equip commercial finite element software ABAQUS with the capability of simulating magneto-mechanical behaviors of ferromagnetic shape memory alloys. A combined convergence scheme is employed to solve the implicit equations. The proposed model together with the presented numerical solution is shown to be able to study shape memory effect and pseudoelasticity at different constant magnetic fields. The simulated magnetic loading/unloading cycles at different constant stresses are found to be well-fitted to the experimental findings. As a practical application of the ferromagnetic shape memory alloy coupled magneto-mechanical response, a spring actuator (a bias spring serially connected to one ferromagnetic shape memory alloy element) is investigated, and the numerical predictions are shown to be in a good agreement with available experimental results. As a novel case, geometrically graded NiMnGa elements are also introduced and are simulated with the use of this approach.

Laser metal deposition of NiTi shape memory alloy on Ti sheet metal: Influence of preheating on dissimilar build-up

Lightweight and functional integration are two major drivers for additive manufacturing technologies. In order to integrate functionality, the use of smart materials like Nickel-Titanium (NiTi) shape memory alloys (SMAs) is a constructive approach. Generally, shape memory alloys are hard to machine and at the same time expensive materials. In this context, additive manufacturing processes like laser metal deposition are reasonable technologies to process these materials as the used powder can be recycled and near net shape geometry can be generated due to a layer-by-layer build-up process. For actuator applications, it might be reasonable to use hybrid systems, meaning just certain sections of a part are made of a shape memory material. One possible example is a NiTi shape memory element on a Ti sheet metal. Due to the varying coefficients of thermal expansion, a dissimilar build-up by means of laser metal deposition without any defects like cracks is challenging. In this paper, an approach that applies preheating is presented to generate SMA elements additively on dissimilar substrates by means of laser metal deposition. Subject of investigations are varying process parameter combinations (e.g., laser power, feed rate, and powder mass flow) and varying preheating temperatures. Based on metallographic analyses, the generated samples are evaluated. Major objective of these analyses are a defect free connection. Additionally, the forming microstructure and the occurring dilution zone are analyzed. In order to characterize the influence of different preheating strategies on the hardness and on the transformations temperatures of the generated parts, the results of hardness measurements and differential scanning calorimetry measurements are presented.

An Overview of Shape Memory Alloy-Coupled Actuators and Robots.

The one-dimensional deformation of shape memory alloy (SMA) wires and springs can be implemented into different types of functional structures with three-dimensional deformations. These structures can be classified based on the type of structure and how the SMA element has been implemented into the following categories: rigid mechanical joints, semi-rigid flexural hinges, SMA elements externally attached to a soft structure, and embedded into the soft structure. These structures have a wide range of properties and implementation requirements, and they have been used to produce a variety of robots with rigid and soft motions. The different research efforts to develop actuators and robots related to each type of structure are presented along with their respective strengths and weaknesses. A model is then developed to discuss the performance and applicability of SMA wires versus SMA springs for actuators with a polymeric matrix to see the effect of each type of SMA on the selection of design parameters. A comparison of the different types of structures and the applicability of different types of SMA elements for different types of structures is then presented.

A review of rotary actuators based on shape memory alloys

The development of rotary actuators is an important aspect of the engineering applications of shape memory alloys. This article reviews about a hundred references on this topic, and presents around eighty actuators driven by shape memory alloys. A classification is made according to the type of rotation (continuous or non-continuous, single or reversible direction). Different factors are then discussed, such as the characteristics of the shape memory alloy elements, the heating and cooling system for the shape memory alloy, the control of the actuator, and the output torque and stroke which can be attained. This article provides the first review focused on rotary actuators triggered by shape memory alloys, highlighting the specificities and potentialities of such actuators for new applications in the future.

Dynamical Behavior of a Pseudoelastic Vibration Absorber Using Shape Memory Alloys

The tuned vibration absorber (TVA) provides vibration reduction of a primary system subjected to external excitation. The idea is to increase the number of system degrees of freedom connecting a secondary system to the primary system. This procedure promotes vibration reduction at its design forcing frequency but two new resonance peaks appear introducing critical behaviors that must be avoided. The use of shape memory alloys (SMAs) can improve the performance of the classical TVA establishing an adaptive TVA (ATVA). This paper deals with the nonlinear dynamics of a passive pseudoelastic tuned vibration absorber with an SMA element. In this regard, a single degree of freedom elastic oscillator is used to represent the primary system, while an extra oscillator with an SMA element represents the secondary system. Temperature dependent behavior of the system allows one to change the system response avoiding undesirable responses. Nevertheless, hysteretic behavior introduces complex characteristics to the system dynamics. The influence of the hysteretic behavior due to stress-induced phase transformation is investigated. The ATVA performance is evaluated by analyzing primary system maximum vibration amplitudes for different forcing amplitudes and frequencies. Numerical simulations establish comparisons of the ATVA results with those obtained from the classical TVA. A parametric study is developed showing the best performance conditions and this information can be useful for design purposes.

Optimization of a Pseudoelastic Absorber for Vibration Mitigation

Dynamic vibration absorbers (DVAs) have received special attention in recent years due to their capability to reduce structural vibrations of a primary structure. In this work, a DVA of the Tuned Mass Damper type based on a Shape Memory Alloy (SMA) element with pseudoelastic behavior is considered. Owing to their rich thermomechanical response, SMAs can exhibit hysteresis loops with rather different features in terms of overall energy dissipation and of pseudoelastic stiffness. As a first step towards the comprehensive evaluation of the performances of such a device, the optimization of a TMD based on SMA devices with different features is studied. Numerical simulations show that the size and the shape of the pseudoelastic loops can influence in a significant way the performances of the DVA.

Effects of heating and cooling cycle on the shape memory and mechanical characteristics of shape memory alloy element for heat-engine.

Effects of heating and cooling cycle on the shape memory and mechanical characteristics of shape memory alloy element for heat-engine.

Adaptive underslung beam using shape-memory alloys for frequency-tuning

The present article addresses the study of an adaptive-passive beam structure with a shape-memory alloy based actuator. In order to mitigate adverse dynamic effects resulting from externally induced vibrations, the structure is able to automatically tune its natural frequency to avoid resonance. The adaptive-passive beam configuration is based on an underslung cable-stayed girder concept. Its frequency tuning is achieved by temperature modulation of the shape-memory alloy elements through a closed-loop control process based on a proportional-integral-derivative algorithm. The effectiveness of the proposed control solution is substantiated by numerical simulations and experimental tests on a small-scale prototype. The validated numerical model enables the simulation of the proposed control approach in a real-scale footbridge, subjected to a prescribed pedestrian loading. The results are very encouraging and show that, by activating the shape-memory alloy elements, the system is able to successfully shift its natural frequency and to mitigate the effects of induced vibrations.

Fiber-reinforced polymers with integrated shape memory alloy actuation: an innovative actuation method for aerodynamic applications

This contribution focuses on the application potential of active fiber-reinforced polymer (FRP) structures with integrated shape memory alloy (SMA) elements for new aerodynamic functions. The advantages of hybrid SMA FRP structures are highlighted, and promising application concepts are discussed. To illustrate the applicability of the technology, the developed manufacturing, design, and control approaches are presented. Main focus is the development of a new concept for an active aerodynamic airfoil and the realization of a hardware demonstrator. Beginning with the idea of an adaptive airfoil, able to bear an application relevant down force at a relatively high deflection, the design process starts with an evaluation of different actuation concepts. An SMA-powered cantilever is a part of the profile itself. Applying the finite-element method with a suitable model for the active hybrid material, an effective selection of material and design is possible. After manufacturing and assembling of the active hybrid airfoil, a comparison of experimental results and simulation is the first proof of success. Finally, the installation of an integrated hardware setup with power source, control, and the active hybrid structure, demonstrating actuation on demand, verifies the potential and the distinct advantages of the new approach using SMA FRP structures.

Analysis of nonlinear free vibration of a beam with magnetic shape memory alloy elements

In this study, nonlinear vibration of a clamped–clamped beam containing magnetic shape memory alloy is investigated through combining the constitutive relations of magnetic shape memory alloy and the large deformation response of Euler–Bernoulli beam. Due to the presence of moderately large strains, the effect of mid-plane stretching is taken into consideration. The magnetic shape memory alloy elements are bounded on the top surfaces in the clamped roots of the beam. Since magnetic shape memory alloy units should be always under compressive stress during the vibration, an appropriate compressive pre-strain is applied in magnetic shape memory alloy elements. In order to derive the governing equation of motion, Hamilton’s principle is utilized, and Galerkin’s method is applied in order to numerically solve the set of nonlinear equations. Variation of the strain in magnetic shape memory alloy elements during the beam vibrations makes it necessary to consider the influence of magnetic shape memory alloy on the beam response as well as the damping effects. Vibration-induced deformation in the beam is transferred to the magnetic shape memory alloys causing reorientation in them. It is shown that as a result of hysteresis behavior of magnetic shape memory alloys, the total energy of the beam dissipates until the amplitude of deformation is low enough that reorientation may no longer be induced in the magnetic shape memory alloys. A detailed study is carried out to investigate the effects of magnetic shape memory alloy elements on the damping as well as the natural frequencies of the beam. The influence of the initial conditions, magnetic field, number of magnetic shape memory alloy elements, and also the pre-strain in magnetic shape memory alloy is numerically investigated.

熱エンジン用形状記憶合金テープ素子の形状記憶・機械的特性に及ぼす加熱冷却サイクルの影響

Since Ti-Ni shape memory alloys(SMAs) work well at temperature less than 373K ,SMAs are expect to apply the heat-engine which recovers low temperature thermal energy .The pulley-type SMA heat-engine, which is one of the SMA heat-engine, is driven by heating and cooling of the liner or coil shape SMA element. We fabricated tape-shaped SMA element for the purpose of improving the product's life span. However, shape memory and mechanical characteristics of tape-shaped SMA is deteriorated with increasing number of heating and cooling cycle. This function deterioration leads to variation of transformation temperatures and shape memory characteristics. The output power of SMA heat-engine depends on the transformation temperature and shape memory characteristics of SMA element. Owing to this, the improvement of function deterioration characteristics of SMA element leads to the output stabilization of SMA heat-engine. In this research, the effects of heating and cooling cycle on the shape memory and mechanical characteristics of tape-shaped SMA element is investigated.

Synergistic use of smart materials for vibration-based energy harvesting

Vibration-based energy harvesting is an approach where available mechanical vibration energy is converted into electrical energy that can be employed for different purposes. This paper deals with the synergistic use of smart materials for energy harvesting purposes. In essence, piezoelectric and shape memory alloys are combined to build an energy harvesting system. The combined effect of these materials can increase the system performance and reduce some limitations. The possibility to control the mechanical stiffness under vibration by a shape memory alloy (SMA) element can provide the ability to tune resonant frequencies in order to increase the output power. The analysis is developed considering a one-degree of freedom mechanical system where the restitution force is provided by an SMA element. The electro-mechanical coupling is provided by a piezoelectric element. Linear piezoelectric constitutive equation is employed together with the Brinson’s model for SMA element. Numerical simulations are carried out showing different responses of the system indicating that the inclusion of the SMA element can be used to extend the operational range of the system.

Modelling Bonded Shape Memory Alloy Vibration Attenuators Elements Using the Finite-Element Method

Shape memory alloys (SMAs) present the capability to develop large forces and displacements with low power consumption. Due their special characteristics, SMAs have been used in many different applications. Pseudoelastic hysteresis loop observed in austenitic SMAs is associated with energy dissipation. Therefore, pseudoelastic SMA elements can be used as vibration attenuators. Joining methods present some technological challenges for the use of these elements. Welding can strongly affect the properties of the alloy. Mechanical joints using rivets and screws are commonly used but promote stress concentration effects. The use of adhesives offers some benefits, being an alternative to be investigated. This work presents a numerical model based on the finite-element method and experimental procedures to study the behaviour of bonded vibration attenuators with SMA elements. The proposed model considers the pseudoelastic behaviour of SMA elements, and a cohesive zone model was used to study the union between absorber and an aluminium plate. Finally, several loading conditions were analysed with the proposed models to assess the capability of bonded pseudoelastic SMA elements to dissipate energy. The proposed geometry allows the elements to actuate as an efficient vibration attenuator, in particular when submitted to axial loading.

Functional properties and morphology of NiTi articulated actuation elements during thermo-mechanical cyclic tests

Shape memory alloys (SMAs) are active materials able to recover large strains over several thermo-mechanical cycles. In the actuation field, this strain recovery is principally exploited on the mini and micro scales at which SMAs exhibit their highest power density with respect to common lightweight technologies. During the repetitive actuation of a SMA element, certain events occur: strain drift at the beginning of cyclic testing, the accumulation of plastic deformation, and strain stabilisation. In this study, these events as well as the overall mechanical response of an articulated NiTi element were monitored through calorimetry and scanning electron microscopy. Fatigue and cyclic stability were tested under different loads and under different aging conditions. In addition, the surface morphology was continuously observed via scanning electron microscopy to monitor crack growth and propagation during the fatigue test. Finally, before and after the fatigue test, samples were tested through calorimetry to investigate the overall microstructural homogeneity. Results confirm the high potential of the proposed geometry for the development of NiTi non-conventional active elements in the miniature actuation field.